1180 
Table 2. Optical [O] and radar [R] remote sensing configurations with high (10) and lower temporal resolution, 
used for the combination method and the accompagnying results, represented by yield errors in tons per hectare. 
Remote sensing data 
category 
Average error (t/ha) 
Without Remote Sensing 
H 
13.4 
CROPSCAN (10) 
[O] 
3.7 
CAESAR (3) 
[0] 
4.2 
AIRSAR L-HH (2) 
m 
9.2 
AIRSAR C-VV (2) 
[R] 
7.2 
AIRSAR L-HH (2) + CAESAR (3) 
[R + O] 
3.0 
AIRSAR C-VV (2) + CAESAR (3) 
[R + O] 
3.5 
4.2 Radar Remote Sensing 
From the MAC Europe campaign 1991 three usable AIRSAR recording dates (July 3rd, July 12th and July 28th) 
are available for the Flevoland test site. The radar backscatter values of sugar beet fields on July 28th were lower 
than those on July 12th, which was attributed to plant structural effects. Therefore, data from July 28th will be 
left out in this study. The Cloud model was successfully calibrated on the radar data of sugar beet fields from 
July 3rd for both L-band HH-polanzation and C-band VV-polarization. However, results with the CAESAR recording 
of July 4th have shown that just one date in the growing season is insufficient for a satisfactory calibration of 
the SUCROS growth model for sugar beet. By lack of ground data and optical data on July 12th (2nd radar 
measurement), the corresponding LAI values were generated by the SUCROS model. It was concluded that the 
parameters of the Cloud model, as given in table 1, for both L-band HH-polarization and C-band VV-polarization, 
respectively, may also be applied to the measurements of July 12th. As a result, we have two data points during 
the growing season for a model-based approach using only radar data. By applying equation (4) with the appropriate 
parameter estimates from table 1, the LAI can be estimated for all sugar beet fields present in both AIRSAR 
images. Equation (5) offers an estimate of the accuracy of these LAI estimates. Subsequently, SUCROS was 
calibrated on these LAI estimates from the AIRSAR recordings of July 3rd and July 12th 1991 for the beet fields 
used before, as far as the corresponding fields were present on both AIRSAR images. Results are given in table 
2 for L-band HH and C-band VV. The comparison between estimated and actual yield is given in figure 5. 
Since we have two recording dates rather early in the growing season, accurate yield estimates cannot be 
expected. On the average, the simulation error of (fresh) beet yield was 9.2 t/ha (13.0% error) for L-band HH 
and 7.2 t/ha (9.8% error) for C-band VV, respectively, with SUCROS calibrated on two AIRSAR dates. This 
is better than the result obtained with "standard" SUCROS without remote sensing information (which gave an 
average simulation error of 13.4 t/ha, see also table 2). Moreover, in this case C-band VV-polarization offers 
better results than L-band HH-polarization. For sugar beet this is about the best we can expect using only the 
model-based approach on radar data, since after mid-July (in 1991) the Cloud model cannot be applied anymore. 
estimated beet yield 
actual beet yield (tons/ha) 
estimated beet yield 
Figure 5. Comparison between estimated yield and actual yield for two AIRSAR recording dates in L-band HH- 
polarization (a) and C-band W-polarization (b).